This invention relates to the soldering of aluminium or aluminium alloys and, more particularly, is concerned with a method of soldering aluminium or aluminium alloys using fluxed solder compositions, and with fluxed solder compositions for use in said method.
There have been proposed a number of solder alloys which are said to be suitable for soldering a member made of aluminium or of an aluminium alloy to another member which latter member may or may not be of aluminium or of an aluminium alloy. These known solder alloys include the so-called "soft solders", which are solders melting below 350.degree. C. The soft solders are generally preferable to the higher melting point "hard solders", and to the "brazing" and "welding" alloys, for any type of joint incorporating a member of aluminium or of an aluminium alloy since with soft solders the expansion, changes of temper and distortion of the member which may take place are less severe at lower temperatures. The use of soft solders also reduces the energy and time required to make a joint, bearing in mind that for aluminium the amounts of energy and time required increase rapidly with soldering temperature since aluminium has a relatively high specific heat. Furthermore, the lower the melting range of the solder used, the wider is the choice of suitable flux compositions which do not char during soldering. The low melting point of a soft solder also permits it to be used on heat-sensitive components such as those found in the electronics and electrical industries.
There are a wide range of metals which are generally recognised by those skilled in the art as being susceptible to soft soldering processes. Such metals include plain carbon steels, brass and copper alloys in general, and nickel alloys. With an appropriate soldering flux, stainless steels can also be soft soldered. Metals such as tungsten, titanium, molybdenum and chromium cannot usually be soft soldered.
Further information concerning the soft soldering of aluminium can be found in
(i) Aluminum Soldering Handbook published by the Aluminum Association in New York in 1971 PA1 (ii) Soldering Aluminium published by the Aluminium Federation in London in 1968.
And in
There are however two disadvantages which may arise from the use of the soft solders heretofore used for soldering aluminium or alloys of aluminium. Firstly, the corrosion resistance of joints made with most of these soft solders is not always satisfactory; failure tends to take place along the interface between the soft solder alloy and the aluminium member so that the electrical and/or mechanical connection across the joint may be lost. Secondly, these soft solder alloys often contain one or more elements (such as bismuth, zinc, cadmium or antimony) which are incompatible with certain other metals, alloys, solders or platings which may form one or more of the other members of a joint comprising an aluminium or aluminium alloy member. For example, antimony attacks zinc in brass, while bismuth, zinc and cadmium are liable to mix with any tin/lead solder which is present to produce a mixed alloy joint of unacceptable composition. National and International specifications for such soldered joints strictly prohibit more than incidental quantities of such elements. Furthermore, soft solder alloys containing one or more of bismuth, zinc, cadmium or antimony may not be ductile enough for production in large quantity in the form of thin solder wire or flux-cored solder wire, may not flow on and wet other members of different metals sufficiently well, and may reduce the corrosion resistance of the joint.
It is an object of the present invention to provide a fluxed soft solder composition suitable for soldering a member of aluminium or of an aluminium alloy to another metallic member, which soft solder composition has a melting point less than 300.degree. C. and can provide a soldered joint with substantial corrosion resistance.